The present invention generally relates to structures used to absorb energy from impact forces applied to the exterior and interior components of motor vehicles. More particularly, this invention pertains to tubular energy absorbing structures that can provide a substantially constant reaction force by progressively deforming and absorbing energy during impact.
The Federal Motor Vehicle Safety Standards (FMVSS) impose performance requirements on automobile bumpers to minimize vehicle damage and on interior trim components to lower the risk of occupant injury caused by vehicle impact. To meet these requirements, automobile manufactures use energy absorbing structures in combination with exterior and interior vehicle components. These structures should be simple, have a low profile, and should be light in weight so as not to significantly affect vehicle performance and fuel consumption. Typical exterior and interior applications for energy absorbing structures in vehicles are identified by locations P in FIG. 11.
Conventional impact energy absorbing structures have included foam structures and flexible tubes having corrugated walls made from laminations of paper, fiber, plastic and/or metal. The tube absorbs energy when the walls of the tube deform in response to external impact forces. Examples of such structures are described in U.S. Pat. Nos. 6,092,555 and 5,680,886, the disclosures of which are incorporated herein by reference. The laminated or layered wall structure can provide enhanced vibration dampening. One example of a prior art wound or spin formed energy absorbing flexible tube is shown in FIGS. 1-3. FIG. 3 is an enlarged view of a wall section, showing inner and outer kraft paper layers 2 with a central metal layer 3.
Unfortunately, prior art tubular structures have not been optimally efficient in absorbing energy produced by external impacts to the vehicle. In this context, the efficiency of energy absorption is determined by analysis of the square wave force (or acceleration) vs. deflection curve. The actual energy absorbed by the tube is represented by the area under force vs. deflection curve. The efficiency of energy absorption is then calculated by dividing the actual energy absorbed by a perfect square or rectangular area, calculated as peak force time vs. deflection.
Ideally, the energy absorbing structure should provide a constant reaction force during impact, i.e., square wave force vs. deflection. In the case of a tubular energy absorbing structure, this means that stresses in the tube walls should be uniform and the tube should uniformly progressively deform (“stroke”) during impact and energy absorption. Unfortunately, prior art tubular structures have not provided a methodology for achieving ideal energy absorbing characteristics.
Energy absorbing structures used in or on motor vehicles should also be easily packaged for shipment and storage, be able to function as a low cost spacer between vehicle fascia covers/trim components and the vehicle body structure and/or have integral fasteners for snap fit attachment to interior vehicle components. Prior art energy absorbing structures cannot provide all of these attributes in combination.